Pattern formation during interfacial reaction in-between liquid Sn and Cu substrates – A simulation study

Abstract

Heteroepitaxial formation of η-Cu6Sn5 intermetallic compound (IMC) on unidirectional Cu has been found to exhibit remarkable microstructural features and provide opportunities for microstructure control of microelectronic interconnects, but the crystallographic origin and formation mechanism of such morphological patterns are still unclear. Here we investigate pattern formation during interfacial reactions in between liquid Sn and two types of unidirectional Cu substrates, (001) and (111), by using a combination of crystallographic analysis, theory of microelasticity and phase-field simulations. The simulation results show that each symmetry-related η-Cu6Sn5 variant grows along the direction of best atomic site matching with the substrates. No spatial alignment can be achieved when the growth takes place without heteroepitaxial constraints. In addition, anisotropy in interfacial energy between the liquid Sn and IMC is found to lead to faceted morphology. Further, the habit planes of η-Cu6Sn5 precipitates and kinematic compatibility across the interfaces are analyzed theoretically and found to be consistent with the simulations. These simulation predictions demonstrate excellent agreements with experimental observations.

A Review on Phase Field Modeling for Formation of η-Cu6Sn5 Intermetallic

“…Generally, the multi-phase field model studying the evolution of microstructure involves three phases, Cu(S), liquid Sn(L), and η-Cu 6 Sn 5 with multiple variants, whose structures are characterized by a set of non-conserved order parameters since the reaction 6Cu(S) + 5Sn(L) → Cu 6 Sn 5 is diffusion controlled. The premise of most phase field modeling is to determine whether the phase transition process can occur, that is, whether the free energies of the reacting phases and their products have intersection in the relative phase diagrams [9,16,[21][22][23][24]. Figure 4 shows the chemical free energy of Cu(S), liquid Sn(L), and η-Cu 6 Sn 5 during reaction 6Cu(S) + 5Sn(L) → Cu 6 Sn 5 under different conditions.…”

“…6Cu(S) + 5Sn(L) → Cu 6 Sn 5 is diffusion controlled. The premise of most phase field eling is to determine whether the phase transition process can occur, that is, whet free energies of the reacting phases and their products have intersection in the r phase diagrams [9,16,[21][22][23][24]. shows the chemical free energy of Cu(S), liquid Sn ( η-Cu6Sn5 during reaction 6Cu(S) + 5Sn(L) → Cu 6 Sn 5 under different conditions.…”

“…Reactions such as 5Cu 3 Sn→9Cu + Cu 6 Sn 5 and 2Cu 3 Sn + 3Sn→Cu 6 Sn 5 may also happen under different aging time and soldering conditions. The formation sequence of η-Cu 6 Sn 5 and ε-Cu 3 Sn is still controversial at present since the experimental observations alone cannot give sufficient information to fully understand the growth kinetics and behavior of IMCs in solder joints [12][13][14][15][16].…”

“…Reactions such as 5Cu3Sn→9Cu + Cu6Sn5 and 2Cu3Sn + 3Sn→Cu6Sn5 may also happen under different aging time and soldering conditions. The formation sequence of η-Cu6Sn5 and ε-Cu3Sn is still controversial at present since the experimental observations alone cannot give sufficient information to fully understand the growth kinetics and behavior of IMCs in solder joints [12][13][14][15][16]. η-Cu6Sn5 possesses a hexagonal structure (space group: P63/mmc) with 𝑎𝑎 𝑛𝑛 = 4.190 Å and it maintains orientation relationships with the Cu pad according to the edge-to-edge matching model.…”

“…η-Cu6Sn5 possesses a hexagonal structure (space group: P63/mmc) with 𝑎𝑎 𝑛𝑛 = 4.190 Å and it maintains orientation relationships with the Cu pad according to the edge-to-edge matching model. The two best matching close-packed planes that contain the rows of atoms are (12 � 10) 𝜂𝜂 ||(11 � 1) 𝐶𝐶𝐶𝐶 and (01 � 10) 𝜂𝜂 ||(100) 𝐶𝐶𝐶𝐶 , as illustrated in [16]. Cu6Sn5 is scallop-type and forms very fast during soldering process.…”